Function (EXT:TYPE-EXPANDtype&OPTIONALonce-p). If type is a user-defined type specifier this will expand it
recursively until it is no longer a user-defined type
(unless once-p is supplied and non-NIL).
Two values are returned - the expansion and an indicator (T or NIL)
of whether the original type was a user-defined type specifier.